1. Field of the Invention
The present invention relates to an apparatus and a method for eliminating radio frequency interference caused by extraneous radar transmission while using a radiometer, and more particularly includes a superheterodyne amplifier and a synchronizing network for isolating radio frequency signals to be measured by the radiometer.
2. Description of the Prior Art
One method for isolating a radar transmitter from the receiving portion of a radar system having a radiometer is the use of a three-port circulator which isolates the transmitter from the receiver due to the directional properties of the circulator. This method adds loss to the radio frequency (RF) input circuit of the radiometer, reducing its effectiveness. Consequently, the isolation is low, generally less than 25 decibels, and the circulator adds volume and cost to the system.
Another method commonly used is a "TR" device, usually a gas discharge tube, or a solid state shorting device. This device is placed in the RF input line and adds losses, volume, and expense.
Still another method which is directed to a combined frequency modulated radar and radiometer system uses a reference system including a dual frequency modulated oscillator which functions as both a radar transmitter and as a radar radiometer local oscillator. Such a reference system includes a switch which alternately connects a receiver to a reference termination and to an antenna. The radiometer used in the reference system is generally of the "Dicke" type modified to include a radar capability. Such a system provides for the broad concept of both a radar and a radiometer capability in combination. However, the ability of the reference system to eliminate radio frequency interference is a disadvantage in some applications.
Another known method is directed to a radar system for detecting low-flying objects. The microwave energy being transmitted is sent simultaneously over a direct path and over a reflected path with the receiver having two radiometers to discriminate between incoming wave energy received over these paths. In this method switching means are used to alternate between cophasal and anti-phasal transmission as well as between additive and subtractive reception over these parts.
The prior art methods described are directed to switching means used in radar systems to optimize data input, and are not directed to the elimination of radio frequency interference.